This invention relates to acoustic conversion devices and, more particularly, to a so-called "edgeless acoustic conversion device" in which no material is used to support its diaphragm.
An ordinary acoustic conversion device such as for instance a loudspeaker has an edge which is made of a material such as paper, cloth or synthetic material which may be the same as or different from the material of its diaphragm, in order to support the latter. However, such an edge material has its own characteristic, and therefore the characteristic of reproduction of a loudspeaker using an edge made of the edge material is affected by the characteristic peculiar to the edge material. This is undoubtedly undesirable.
As the amount of absorption loss of the edge is considerably large, the provision of the edge using such a material is undesirable in order to reproduce the variations of an input signal with high fidelity. That is, for this purpose, a so-called "edgeless acoustic conversion device" which is operated through air is suitable.
This kind of loudspeaker is known as the edgeless acoustic speaker. The edge portion of the loudspeaker is as shown in FIG. 1. The diaphragm 10 of the loudspeaker comprises: a cone part 10a whose configuration is conical; and a peripheral part 10b forming a predetermined angle .alpha. with the cone part 10a. The frame 11 of the loudspeaker is confronted with the peripheral part 10b is such a manner that there is a certain gap 12 between the frame 11 and the peripheral part 10b. The gap 12 is acted as a so-called "acoustic low-pass filter." The function of the gap 12 is similar to that of the duct of a bass reflex type loudspeaker. That is, the gap 12 is adapted to positively facilitate the flow of air caused by the vibration of the diaphragm 10, thereby to decrease the lowest resonance frequency of the loudspeaker.
In the case where the conventional loudspeaker thus constructed is used with a bass reflex type loudspeaker box, the acoustic impedance of the gap can be neglected when compared with that of the duct, and therefore the flow of air around the edge can be disregarded.
However, when the conventional loudspeaker is mounted on a closed box, the gap acts similarly as in the duct of the bass reflex type loudspeaker as described above. This is not proper in the case where it is not positively intended to allow the flow of air in the gap. This difficulty attributes to the fact that the conventional edgeless type loudspeaker is different from an ordinary loudspeaker which is so modified that its edge is supported by air resistance; that is, its support is effected only by the damper.